1. Field Of The Invention
This invention relates to a low drag surfaces for use in flowing fluids, and more particularly, to such surfaces having a contoured surface specifically designed to reduce drag compared to the drag associated with a corresponding smooth surface.
Many surfaces are designed to be placed in a flowing fluid. For example, airplanes, boats, compressor blades and many other objects are specifically designed to have a low drag when there is relative flow between that object and the environmental fluid in which it operates. In all cases, the drag relates to the viscosity of the fluid and the relative velocity of the fluid with respect to the object. Drag, is a force in the direction of the relative fluid flow and opposite to the relative motion of the object, that causes either a limited velocity for a constant force or the need for additional force to be provided to overcome the effects of the drag and maintain a desired velocity.
2. Description Of The Prior Art
In the past, designers of objects, which are designed to operate in a flowing fluid environment, have attempted to make the surface as smooth as possible in order to reduce the drag. For example, rivets used in airplanes and submarines to hold the skin covering the structural supports are normally ground to mate as evenly as possible with the surrounding surfaces of the skin.
However, in the past, several others have suggested modifying the generally smooth surfaces of members subject to fluid flow in order to reduce drag, or for other similar reasons. For example, reference is made to U.S. Pat. No. 1,454,479 in the name of D. R. McCullough and entitled, "Airplane", where small Vee shaped projections along the streamline are described for the purpose of providing additional lifting capability for the airplane. However, the McCullough structure results in a vortex behind each extension and thereby has a net effect of increasing the drag. In U.S. Pat. No. 1,726,882, in the name of A. Boerner and entitled, "Means For Overcoming Fluid Friction", small openings in the body of an airplane or ship are provided to permit fluid to pass therethrough and be ejected through downstream openings. Boerner also has a series of ridges in the walls and suffers the same problem that McCullough suffers in that the small vortex created behind each ridge which increased the overall drag. Boerner also suggests fluid driving means be provided between the upstream and downstream openings for providing further thrust, which is necessary to overcome the added drag.
In U.S. Pat. No. 2,608,171, in the name of I. H. Pearce and entitled "Corrugated Air Distributing Underbody For Water Borne Vessels", there is suggested the addition of longitudinal ribs to the outer surface of a water craft and the introduction of air between the ribs to maintain the water remote from the hull of the craft. This structure does in fact assist in reducing drag to a limited extent, but fails to take into account all of the various causes for drag. In U.S. Pat. No. 2,800,291, in the name of A. V. Stephens and entitled, "Solid Boundary Surface For Contact With A Relatively Moving Fluid Medium", it is suggested that triangular projections extend upward in a downstream direction for the purpose of delaying or preventing the separation of the fluid flowing around an airfoil. While the structure suggested by Stephens may accomplish its intended purpose of delaying the separation, it does not sufficiently delay the separation to the end of the airfoil.
In U.S. Pat. No. 2,899,150, in the name of F. E. Ellis, Jr. and entitled, "Bound Vortex Skin", the inventor suggests providing a series of spanline longitudinal cylindrical grooves along the wing of an airplane for the purpose of providing additional lift. This structure, however, greatly increases the drag with a net negative effect. In U.S. Pat. No. 3,184,184, in the name of H. A. Dorman et al and entitled, "Aircraft Having Wings With Dimpled Surfaces", dimples, such as found on a golf ball, are provided on the wings of an aircraft for the purpose of reducing drag. Unfortunately, these dimples do not reduce drag in an airfoil in the same manner as they allow a golf ball to rise, as contemplated by Dorman et al, because the airfoil does not spin as does a golf ball. U.S. Pat. No. 3,213,819, in the name of G. May and entitled, "Boundary Layer Control Assembly", apparatus and a technique to fabricate ribbon having periodic notches therein is disclosed for the purpose of fabricating porous skins for devices subject to relative fluid flow.
In U.S. Pat. No. 4,284,302, in the name of H. F. P. Drews and entitled, "Driven Craft Having Surface Means For Increasing Propulsion Efficiencies", a pattern of pointed upward extensions of various types are shown on an automobile for the purpose of reducing drag. This structures suffers the same problems as McCullough, discussed above. In U.S. Pat. No. 4,354,648, in the name of K. M. Schenk et al and entitled, "Airstream Modification Device", a series of spanline extension elements are placed on the surface of an airfoil for introducing multi-directional turbulence for the purpose of delaying the airstream separation from the airfoil. While the Schenk et al structure accomplishes what it intends, it does so by increasing turbulence and hence drag. In U.S. Pat. No. 4,434,957, in the name of R. R. Moritz and entitled "Low Drag Surface", an airfoil having a surface configuration to produce a vortex and control the subsequent movement of that vortex for the purpose of reducing drag is suggested. While this was a popular theory several years ago, no evidence exists to support the Moritz theory and in fact, it simply does not produce any beneficial results.
In U.S. Pat. No. 4,650,138, in the name of R. D. Grose and entitled, "Cascaded Micro-Grove Aerodynamic Drag Reducer", an airfoil is disclosed having streamline directed grooves therein, including micro-grooves within each of the grooves. In this regard, the Grose structure is similar to the Pearce structure, and does provide some benefit in reducing drag, but does not take into account the other factors necessary to better reduce drag. In U.S. Pat. No. 4,750,693, in the name of G. Lobert et al and entitled, "Device For Reducing The Frictional Drag Of Moving Bodies", a series of spanline microgrooves are provided to reduce drag on an airfoil. This structure does reduce drag to a limited extent, but it is very difficult to place a series of 0.3 micron grooves throughout the surface of an airplane or boat hull. Thus, the Lobert et al structure is not very practical.
Thus, none of the inventions described in the prior art patents operates to sufficiently reduce drag in a surface having fluid flowing thereacross, as indicated by the lack of widespread use of such inventions. One mistake of the prior art patents is that no single structure deformity is useful in all circumstances to adequately reduce drag. As noted above, drag relates to the viscosity of the fluid and the relative velocity of the fluid with respect to the object. However, drag also relates to the turbulence caused by the object relative to the moving fluid. Thus, one way to reduce drag is to reduce the turbulence created by the object.